1. Field of the Invention
This invention relates generally to a protective coating composition, and more particularly to an abrasion-resistant, heat-resistant and weather resistant coating composition for metallized or nonmetallized surfaces. In particular, the present invention relates to a modified silicone resin composition which, when coated, for example, onto vacuum metallized or vacuum sputtered plastic surfaces, provides a transparent protective barrier to weathering forces.
2. Description of the Related Art
The commercial advent of solar concentrating devices that intensify solar radiation via parabolically curved or planar reflective surfaces has created interest in low cost, lightweight, weatherable, and durable plastic mirrors. Such mirrors or reflectors also are desired for other uses, for example, as reflectors for fluorescent lamps.
Such plastic mirrors can be produced by vacuum metallizing aluminum or some other metal onto a clear plastic film such as polyester, then applying via roll coating (gravure, reverse roll, etc.) a thin layer of a clear plastic solution, then force air drying for fifteen to sixty seconds at a relatively low temperature of approximately 125 degrees Centigrade. At the end of the drying period, the coated film is wound onto a roll, either with or without a protective slip sheet.
To implement the use of a plastic film reflective surface, the opposite (uncoated) surface may be subsequently coated with an adhesive thereby enabling the coated composite to be adhered to either a curved or flat metal surface or to be laminated to a second plastic film or surface.
The properties required in such a protective coating demand a material that, when coated and dried, will not block or even more stringently, will not optically mar when rolled up. The optical mar referred to can be any form of surface hazing, dulling, marring, or picking, each of which in varying degree, will diminish the reflectivity and particularly, the specular reflectivity of the surface. In addition, in the likely event that an adhesive will be applied to the opposite surface, it is of critical importance that the protective coating is capable of resisting the effects of heat and pressure as it will be in direct contact with the heated metal or plastic rolls of the coating machine. Therefore, a coating with deformation resistance to temperatures of 125.degree. to 150.degree. C. is necessary.
Thermoplastic acrylic resins, which do have good weathering properties, have been usefully employed for many years in coating metal surfaces. However, these resins soften too readily under the conditions described above, producing a reflective surface which is optically marred.
Also, the prior art is replete with a variety of compositions including a silicon-containing compound such as a silane and/or a polyisocyanate compound, which are described as being useful for coatings, sealants, lacquers and the like. These compositions are stated to impart properties including one or more of the following: clarity, adherency to various substrates, abrasion resistance, ultraviolet ray resistance, fogging resistance, resistance to moisture and humidity, weather resistance and resistance to discoloration by heat.
U.S. Pat. No. 4,006,271 to French, et al. thus discloses an abrasion-resistant adherent coating for polycarbonate substrates such as lenses. The coating contains a hydrolyzed C.sub.1 -C.sub.2 alkyltri- (lower alkoxy) silane in a water-miscible organic solvent, an amino alkoxy silane contributing additional abrasion resistance and a stabilizing weak acid.
Japanese Patent No. 52-17519 to DeSantis discloses a sealing composition composed of a mixture of a solution of polyester resin in solvent, a reaction product of a polyisocyanate and a silane containing active hydrogen, a mixed isocyanurate polyisocyanate (prepared from TDI and MDI) and a catalyst.
U.S. Pat. No. 4,098,840 to Yoshida et al. discloses a thermosetting resin composition which forms a transparent, anti-fogging, abrasion resistant coating for the surfaces of inorganic glass, metals and synthetic resins. The composition comprises a hydrolysate of an alkoxysilane and a polymer or copolymer of certain monomers such as, for example,
hydroxyethyl methacrylate or acrylate,
U.S. Pat. No. 4,369,300 to Carter discloses a radiation curable coating. Acrylated urethane silicone compositions formed from the reaction of a silicone carbinol, a polyisocyanate and a hydroxyfunctional acrylate are disclosed.
U.S. Pat. No. 4,472,464 to Haluska et al. discloses the use of novel silanes in curable coatings. This allows such curable coatings, as a result, to be receptive to tinting or dyeing, after being cured on particular substrates.
U.S. Pat. No. 4,476,281 to Vaughn discloses a silicone resin coating which, when applied to solid substrates, forms a protective abrasion-resistant coating. The coating composition, prepared by hydrolyzing a methyl trimethoxy silane in an aqueous dispersion of colloidal silica, is described as adhering well to metals and metallized surfaces with resistance to moisture, humidity and ultraviolet light. Similar compositions are disclosed in U.S. Patent No. 3,986,997 to Clark, U.S. Pat. No. 4,177,315 to Ubersax, U.S. Pat. No. 4,277,287 to Frye, and U.S. Pat. No. 4,482,656 to Nguyen et al.
U.S. Pat. Nos. 3,707,521 and 3,779,794 to DeSantis disclose a polyurethane sealant-primer system comprising an isocyanate reactive surface primer composition and a moisture-curable polyurethane sealant composition.
U.S. Pat. No. 4,482,656 to Nguyen discloses a method for grafting a hydrolyzed alkoxy silane onto colloidal particles of silica or alumina, thereby transforming an inherently hydrophilic mineral surface into one which is hydrophobic. The grafted particles are then used in coatings to provide abrasion resistance.
U.S. Pat. No. 4,288,586 to Bock discloses the preparation of polyisocyanates containing isocyanurate groups by the partial trimerization of the isocyanate groups of isophorone diisocyanate (IPDI) using catalysts which accelerate the trimerization of isocyanate groups. The catalysts used are quaternary hydroxyalkyl ammonium hydroxides containing at least one hydroxyalkyl group. These polyisocyanates, unblocked or blocked by known techniques, may be used to prepare lacquer binders. The resulting lacquers give rise to films which are said to adhere surprisingly firmly to metal surfaces, are exceptionally light-fast, resistant to discoloration and highly abrasion resistant. In addition, it is stated that these lacquers are distinguished by great hardness, elasticity and chemical resistance, high gloss, excellent weather resistance and a good pigment affinity.
Curable isocyanate prepolymers, in which all or a portion of the available isocyanate terminal groups are reacted with a secondary amine containing silane monomer having two trialkoxy silane groups, are disclosed in U.S. Pat. No. 4,374,237 to Berger. Such prepolymers are useful to curable sealant compositions having enhanced wet adhesion combined with the desirable properties of polyurethanes, e.g.--tear resistance, extensibility and elastic recovery.
Despite all the prior work, the several compositions described fail to provide a composition which imparts the diverse properties required to serve as a desirable protective coating for metallized and sputtered substrates. There is accordingly a clear need to provide a composition satisfying the necessary properties for this type of application.